Police officers often ask motorists suspected of drunk or drugged driving in Oklahoma to perform a series of exercises that are generally referred to as a standardized field sobriety test. These tests have been designed by experts to reveal signs of impairment, and strict protocols must be followed when they are conducted. Drivers are asked to walk and turn, raise one foot off the ground and follow an object, such as a pen, with their eyes. When performed correctly, field sobriety tests are can identify severe intoxication about 80% of the time.

Drivers are asked to follow an object with their eyes to measure their horizontal gaze nystagmus, which is the involuntary movement the eye makes when it moves to the side. This movement becomes far more pronounced when individuals consume alcohol or prescription drugs that depress the nervous system. Studies funded by the National Highway Traffic Safety Administration have revealed that HGN evaluation is the most effective part of a standardized field sobriety test.

During an HGN evaluation, police officers look for jerking movements as the eye moves from side to side, and they pay particular attention to how the eye behaves when it focuses on a peripheral item for four seconds. While exaggerated eye movement during an HGN evaluation is a reliable sign of intoxication, it can also be caused by a brain injury, eye strain or fluid moving in the inner ear.

Experienced criminal defense attorneys may study arrest reports carefully when their clients face drunk driving charges after failing a standardized field sobriety test. When it can be established that a field sobriety test was administered improperly or the police officer involved lied about how they conducted the test, attorneys may seek to have DUI charges dismissed.